A diaphragm-type pressure sensor device has a sensor chip in which, schematically, a strain resistance gauge is formed on a laminated diaphragm made of silicon, glass or the like. Based on a change of a resistance value of the strain resistance gauge formed in the diaphragm that is displaced by pressure, the pressure sensor device detects the pressure applied to the diaphragm.
For example, a pressure sensor device of this type is configured by installing a sensor chip 1 having a diaphragm, for example, into a meter body 2 shown in FIG. 13. The meter body 2 has barrier diaphragms 4a and 4b forming a pair of pressure-receiving portions in a body portion 3 thereof. The meter body 2 has a structure in which the sensor chip (pressure sensor) 1 installed in a sensor portion 5 communicates with the barrier diaphragms 4a and 4b through pressure-absorbing chambers 7a and 7b, respectively, which are separated by a center diaphragm 6 with a large diameter. A pressure-transmitting medium such as silicone oil is injected into connecting paths 8a and 8b that connect the sensor chip 1 and the barrier diaphragms (pressure-receiving portions) 4a and 4b. 
With the meter body 2 thus configured, as FIG. 14 schematically shows an aspect of the operation thereof, pressures Pa and Pb applied to the barrier diaphragms 4a and 4b are led to both sides of a diaphragm, not shown, of the sensor chip 1 through the pressure-absorbing chambers 7a and 7b separated by the center diaphragm 6 in a steady state. As a result, the diaphragm of the sensor chip (pressure sensor) 1 shows displacement corresponding to differential pressure between the pressures Pa and Pb, that is, ΔP(=Pa−Pb).
On the other hand, if either of the barrier diaphragms 4a and 4b is applied with maximum pressure Pover, the barrier diaphragm 4a is greatly displaced, for example, as shown in FIG. 15. In response to the great displacement of the barrier diaphragm 4a, the center diaphragm 6 is so displaced as to absorb the maximum pressure Pover. When the barrier diaphragm 4a reaches the bottom of the body portion 3, and the displacement thereof is restricted, this hinders further transmission of the pressure P through the barrier diaphragm 4a. Consequently, the sensor chip 1 is prevented from being damaged by application of the maximum pressure Pover. The sensor chip 1 detects only the differential pressure ΔP between the pressures Pa and Pb applied to the pair of barrier diaphragms 4a and 4b. 
The pressure sensor device in which the sensor chip 1 is installed in the meter body 2 has the barrier diaphragms 4a and 4b and the center diaphragm 6 as mentioned. The pressure sensor device has a configuration in which the sensor chip 1 is protected from the maximum pressure Pover by functions of the diaphragms 4a, 4b and 6. Therefore, it is undeniable that the entire shape of the pressure sensor device including the meter body 2 is large-scaled. In this respect, there is not only a demand for downsizing of the pressure sensor device but also an intense demand for sufficient increase of operating pressure for protection against maximum pressure (withstand pressure) of the pressure sensor device.
In order to prevent accidental damage or breakdowns of diaphragms, for example, Unexamined Japanese Patent Publication No. 10-78366 proposes that a backup (stopper) portion be so disposed as to face a diaphragm of the sensor chip 1 with the prescribed space therebetween, to thereby prevent excessive displacement of the diaphragm in the backup (stopper) portion. Specifically, the publication describes that if the backup portion is formed into a stepped concave surface, even when the diaphragm is displaced by application of the maximum pressure, stress concentration does not occur in the rim portion of the diaphragm.
However, even if the stepped backup (stopper) portion is so disposed as to face the diaphragm, this merely increases the operating pressure for protection against maximum pressure severalfold at most, as compared to the withstand pressure possessed by the sensor chip 1 itself. Accordingly, in order to improve the detection sensitivity with respect to the differential pressure ΔP and set the operating pressure for protection against maximum pressure at sufficiently high degree, it is essential in general to use the meter body 2 of the above-described structure. Consequently, the necessity of the meter body 2 is a great issue in consideration of downsizing of the pressure sensor device.